$\begingroup$Briefly: yes, it's possible for any polyprotic acid, using pH and Ka only; no, the formula for DD seems to be incorrect. If you read German, the Wikipedia article covers it all: de.wikipedia.org/wiki/Dissoziationsgrad. If not, I can put some sort of an answer based on that in a couple of hours.$\endgroup$
– andselisk♦Jan 10 at 13:17

$\begingroup$I can't read german, but thanks. I got this equation from ostwalds law and pH = 1.2pka - 1/2log ca. However, this is probably only valid when the acid is considered very weak?$\endgroup$
– delivosaJan 10 at 13:50

2 Answers
2

Even though the degree of dissociation $α$ depends both on the nature of the dissolved electrolyte (e.g. acid) and the concentration, for the homogeneous medium it is possible to determine $α$ by $\mathrm{pH}$ and $\mathrm{p}K_\mathrm{a}$ only, without any auxiliary information such as initial concentration since $\mathrm{pH}$ is a function of concentration.

I would suggest you not to remember certain formulas, since they have been derived only for particular systems and cannot be universally applied. Instead, use the law of mass action, especially when it comes to multi-component systems and multi-step dissociating electrolytes.

where $n\in\mathbb{N}$, $m\in\mathbb{N}$, $m < n$; $K_{\mathrm{a}m}$ – $m$-th dissociation constant. Dissociation degree $α$ for any component of the composition $\ce{H_{n-m}A^m-}$ formed at the $m$-th step is by definition

$\begingroup$Pragmatically I think that this also means that pH < 5 or pH > 9 to avoid complications due to autodissociation of water (assuming that you want 1% accuracy...)$\endgroup$
– MaxWJan 10 at 21:49

1

$\begingroup$Thank you so much! This is such a clear answer.$\endgroup$
– delivosaJan 12 at 8:58

$\begingroup$And is this only valid if you say that the acid is very weak? So doesn't dissociate very much?$\endgroup$
– delivosaJan 10 at 13:40

$\begingroup$It is valid for all acids, but (very) strong acids will for all practical purposes be completely dissociated (Ya=1). But it is only valid for diluted solutions. When the concentration is too high the calculated result will deviate.$\endgroup$
– FrankSJan 10 at 14:38

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$\begingroup$The equation isn't valid for dilute solutions of a weak acid where the autoionization of water would need to be considered.$\endgroup$
– MaxWJan 10 at 21:52